1. Field of the Invention
The present invention relates to a turbo compressor capable of compressing fluids using a plurality of impellers, and a refrigerator equipped with the turbo compressor.
Priority is claimed on Japanese Patent Application No. 2008-027069, filed Feb. 6, 2008, the content of which is incorporated herein by reference.
2. Description of the Related Art
As a refrigerator that cools or refrigerates an object to be cooled such as water, a turbo refrigerator or the like is known which is equipped with a turbo compressor that compresses and discharges a refrigerant by a compression device provided with an impeller or the like.
In the compressor, a higher compression ratio leads to a higher discharge temperature and a lower volumetric efficiency of the compressor. Accordingly, in the turbo compressor as mentioned above which is installed in the turbo refrigerator or the like, it is necessary, in some cases, to conduct the refrigerant compression through multiple stages. For example, in Japanese Unexamined Patent Application, First Publication No. 2007-177695, a turbo compressor is disclosed which has two compression stages, each of which is equipped with an impeller and a diffuser, and compresses a refrigerant sequentially through these compression stages.
In addition, in such a turbo compressor, an oil tank is provided which stores a lubricating oil to be supplied to the sliding portion in the compression device. In this oil tank, in order to recover the lubricating oil supplied to the sliding portion, it is necessary to create a pressure gradient so that the internal pressure is lower than that of the space where the sliding portion is located.
Accordingly, in the conventional turbo compressors, the pressure inside the oil tank has been made negative to recover the lubricating oil by directly connecting the oil tank and a suction port of the compression device via a piping (a pressure equalizer) so that the pressure inside the oil tank equals to that of the suction port, which has the lowest pressure in the compression device.
Meanwhile, the conventional turbo compressors as described above have been associated with the following problems.
That is, when operating a compressor, the pressure inside the oil tank reduces rapidly as the gas in the compressor is suctioned, since the oil tank and the suction port of the compression device are directly connected via a pressure equalizer. As a result, the gases which have been dissolved in the lubricating oil such as a refrigerant gas vaporize, resulting in what is known as oil foaming. Due to this oil foaming, the mist of oil filling inside the oil tank flows into the suction port through the pressure equalizer. For this reason, the amount of lubricating oil reduces which results in an insufficient supply of the lubricating oil to the sliding portion, and also the mist of oil mixes with the gas suctioned in by the compressor which results in the deterioration of compression properties.
The present invention is made in view of the above circumstances and its object is to provide a turbo compressor and a refrigerator which enable the recovery of lubricating oil by making the pressure inside the oil tank negative, while preventing the reduction of lubricating oil and the deterioration of compression properties.